The present invention relates to a process for imaging a continuous web of printing plate stock in a single web processing line having a plurality of imaging stations. In the preferred embodiment, the web is prepared to be imageable by either of two or more separate imaging modes. The process employs a continuous web imaging line having at least two imaging stations in the line, one station operating in a first imaging mode and the other preferably operating in a second imaging mode. Specifically, the invention relates to lithographic printing plates.
Conventional lithographic printing plates, such as those typically used by both newspaper and commercial printers, are usually made of a grained, anodized aluminum substrate which has been coated with a light sensitive coating. The anodized aluminum is generally post treated to enhance the hydrophilicity of the anodized substrate sheet prior to the application of the light sensitive coating.
Graining of the aluminum is commonly accomplished in a variety of ways including mechanical brush graining, chemical graining and electrochemical graining. The grained surface has better adhesion to the light sensitive coating and carries fountain solution in the background areas of the plate on the press more efficiently than an ungrained surface. Anodizing is the process of electrolytically generating aluminum oxide on the surface of an aluminum sheet. Since the anodic aluminum oxide is harder and more abrasion resistant than the aluminum, an anodized printing plate has a greater press life than a bare plate.
Conventional lithographic printing plates with a positive working light sensitive coating are imaged with actinic radiation, such as ultra violet light, through a photographic positive to solubilize the light sensitive coating in the non-image areas. The plate is then developed with a solvent for the solubilized coating. With such positive working plates, the coating remains in the areas which have not been exposed to the light and it is these areas which are oleophilic and comprise the image to be printed.
Lithographic printing plates are also available which are imageable by the use of infrared lasers. For example, U.S. Pat. No. 4,731,317 to Fromson et al., discloses a printing plate based on a substrate which is brush grained in a slurry comprising alumina followed by successive treatments in dilute sodium hydroxide and nitric acid, and subsequent anodizing to achieve an oxide coating weight of 1.5 milligrams per square inch. The substrate may also be silicated after anodizing to improve hydrophilicity. The anodized plate is coated with a diazo resin which is transparent to the radiation of a YAG infrared laser (1064 nanometers), but is sensitive to the longer wavelengths generated within the areas of the anodic oxide exposed to the laser. In this case, the plate is negative working since the diazo is rendered insoluble where the plate is exposed to the laser. Following laser exposure, the unexposed diazo is removed with a solvent to reveal the hydrophilic oxide.
Several options are available for forming an image on a lithographic plate. Digital imaging by a computer controlled laser is a relatively new technology that eliminates the need for imaging and processing film. The digital imaging can take place as a result of selective ablation, curing or photolytic alteration of the solubility of the coating. At the present time, the cost of digital plates is typically higher than for conventional analog type plates. Another alternative imaging technique involves the use of electrostatic ink jet deposition technology. There are again several approaches. One can ink jet an oleophilic material directly onto a hydrophilic substrate. An ink jet material can be coated onto a photosensitive coating and serve as a photomask for actinic exposure in either a positive or negative mode. U.S. Pat. No. 5,750,314 to Fromson et al discloses an insoluble ink jet coating selectively applied over a soluble lithographic coating which need not be photosensitive. The insoluble ink jet mask prevents the soluble coating from being removed by the solvent. Each of these imaging techniques has a particular advantage. It would, therefore, be desirable to have different types of imagers available without consuming excess space in the printing plant.
Another factor in the conventional production of lithographic printing plates is that the typical process involves the handling of a stock of individual sensitized plates with all of the required handling time and equipment. Each plate must be loaded into an imager, imaged and then unloaded. Imaging the printing plates in a continuous process line on a web including accurate registration and cutting the imaged plates from the web to form the individual plates would greatly simplify the process. Therefore, such imaging on a continuous web is the subject matter of the copending U.S. patent applications Ser. No. 09/041,615 of Fromson et al. filed Mar. 12, 1998, now U.S. Pat. No. 5,865,118 and 08/955,041 of Fromson et al. filed Oct. 21, 1997 now U.S. Pat. No. 5,826,513.